An organic electroluminescent device (hereinafter “electroluminescent” is abbreviated as “EL”) has a structure wherein an organic layer formed by stacking an organic carrier transporting layer and an organic emitting layer is arranged between first and second electrodes. Attention is paid to the device as a self-emission type display element which can emit light with a high luminance by low voltage direct current driving.
The cross section structure of the organic EL device is categorized as a bottom-emission structure or a top-emission structure according to a light-outcoupling direction.
In the top-emission structure, light is outcoupled from the side opposite to a substrate, differing from a conventional structure (bottom emission) in which light is outcoupled through a glass substrate supporting an organic EL device. This improves the aperture ratio in an emitting part, whereby the luminance can be increased.
A method has been studied in which a semitransparent cathode is used as an upper electrode and only light having a specific wavelength is intensified by utilizing a multiple interference effect and outcoupled from the EL device to achieve high color reproducibility.
For example, Patent document 1 discloses an organic EL device in which a first electrode formed of a light-reflecting material, an organic layer including an organic emitting layer, and a second electrode formed of a semitransparent reflecting layer and a transparent material are stacked such that the organic layer serves as a resonator, and which device satisfies the following formula (1) when λ is the peak wavelength of the spectrum of light intended to be outcoupled.(2 L)/λ+φ/(2π)=m  (1)wherein L is an optical length, λ is a wavelength of light intended to be outcoupled, m is an integer, φ is a phase shift in electrodes, and the optical length L is adjusted to be a positive minimum.
There is disclosed a full color display formed by combining such a top-emission type organic EL device and a thin film transistor (TFT) array for actively driving this organic EL device (see Patent document 2, for example).
In the technique disclosed in Patent document 1, since the organic layer provided between the first and second electrodes has a small thickness, a short circuit between the two electrodes is caused by influence of surface unevenness of the first electrode, reducing a fabrication yield of the organic EL device.
Specifically, FIG. 4 shows the relationship between the thickness of the organic layer and m of the formula (1).
FIG. 4 shows the relationship between the thickness of the organic layer (x+50) (nm) and m of the formula (1) for light having a wavelength of 455 nm (B), 520 nm (G) and 620 nm (R) in the organic EL device of aluminum/ITO (10 nm)/hole transporting layer (x nm)/emitting layer (30 nm)/electron transporting layer (20 nm)/Mg—Ag alloy layer (10 nm)/ITO (100 nm). The relationship was obtained by calculation.
FIG. 4 shows that a short circuit between the two electrodes tends to occur due to the thin thickness of the organic layer of 100 nm or less in the case of light having a short wavelength of 520 nm or less.
As a technique for preventing a short circuit between electrodes, there is known a technique of smoothing the surface of a first electrode by polishing. This is effective for a bottom-emission type device. However, when the technique is applied to a top emission-type device, in particular a device where a top-emission structure is provided on the upper part of a TFT alley as shown in Patent document 2, static electricity caused by polishing may break the TFT alley part.
As another technique for preventing a short circuit between electrodes, an organic layer is thickened. If the thickness is selected such that m is 1 in FIG. 4, a short circuit can be prevented while reinforcing the intensity of light by utilizing a multiple interference effect.
However, a luminous efficiency decreases in the case of the thick film of m=1 compared with the case of the thin film of m=0.
FIG. 5 shows the relationship between a light-outcoupling efficiency (ratio of light outcoupled from an organic EL device to the outside) and the thickness of a hole transporting layer. Specifically, FIG. 5 shows the relationship between a light-outcoupling efficiency for light having a wavelength of 455 nm emitted from an emitting layer and the thickness x of a hole transporting layer in an organic EL device of aluminum/ITO (10 nm)/hole transporting layer (x nm)/emitting layer (30 nm)/electron transporting layer (20 nm)/Mg—Ag alloy layer (10 nm)/ITO (100 nm). The relationship was obtained by calculation.
FIG. 5 shows that the light-outcoupling efficiency becomes maximum when the hole transporting layer has a thickness of 36 nm (m of the formula (1) is 0) or 152 nm (m of the formula (1) is 1). The two maximum values are substantially the same. That is, the light-outcoupling efficiencies are almost the same for the specific wavelengths. However, a high light-outcoupling efficiency is required over a relatively wide wavelength region because light emitted from an organic EL device generally has a broad spectrum.
FIG. 6 is a graph showing the wavelength dependency of light-outcoupling efficiency and the emission spectrum of organic EL device.
Specifically, FIG. 6 shows the wavelength dependency of light-outcoupling efficiency in the two cases where a hole transporting layer has thicknesses of 36 nm (m=0) and 152 nm (m=1) in the organic EL device of FIG. 5.
Apparently from FIG. 6, the full width at half maximum of the light-outcoupling efficiency in the case of thick film of m=1 is narrower than that in the case of thin film of m=0. An organic EL device generally emits light with a broad emission spectrum. The full width at half maximum in the case of thick film of m=1 is narrower than that of the emission spectrum of organic EL device. This means that light of the shaded region in FIG. 6 can not be outcoupled from the device to the outside, whereby the luminous efficiency is reduced.
As described above, an organic EL device having a high luminous efficiency can not be obtained only by thickening its organic layer.    [Patent document 1] WO01/39554    [Patent document 2] JP-A-2001-195008
In view of the above-described problems, an object of the invention is to provide an organic EL device having a high luminous efficiency while preventing a short circuit between a first electrode and a second electrode.